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walled tracheids have a similar function and rank next in import- 
ance, those at least whose walls are so thin that they are of little 
use as strengthening cells, and which differ from the real trachez 
only by the lack of direct communication with each other, and by 
their usually smaller diameter. It is well known that some of these 
tracheids are supplied with peculiar shaped pores whose function 
is generally conceded to be that of facilitating the passage of some 
part of the cell contents from one cellto another. Next we come to 
the tissue forming the subject of the present work, namely : the lib- 
riform or thick walled woody tissue. In many instances the cells 
of this tissue are found to possess pores of the same peculiar 
shape as those of the thin-walled tracheids. Hence a slight ground 
for the inference that these cells also serve to a certain extent as 
water carriers and rank next in importance to the tracheids, the 
passage of the water being facilitated by the same mechanical 
construction as in the cells of the latter class. Lastly we come 
to the remaining cells of the libriform tissue, which are supposed 
to be of the least importance in this respect, as they are supplied 
only with simple pores, such as are found in all parts of plant- 
organs wherever thick-walled cells occur, and where there is no 
reason for supposing any arrangement specially adapted to the 
rapid transport of water. 
A second and perhaps better reason for this assumption, is 
the result of various experiments with the wood of coniferous 
trees, which contains no trachee nor thin walled tracheids, but 
with the exception of the cells of the medullary ray, consists en- 
tirely of thick walled tracheids abundantly supplied with these 
peculiar pores. The experiments of Robert Hartig and others 
have proved that no considerable amount of water passes through 
these cell walls, therefore furnishing strong presumptive evidence 
in favor of the assumption that the passage of water upward from 
one cell to another, in the case of the tracheids of the conifers, is 
in some way facilitated by these bordered* pores, which as before 
stated, correspond exactly in form with the bordered pores of the 
libriform tissue of dicotyledonous woods. 
* The word bordered is used here in the sense of the German word behéft, and 
throughout this article the word border is used for the German word hof, when 
= pests to the widening at the extremity of the pore next the outer surface of 
cell wall. : 
